Non-Fermi-Liquid Behavior of Superconducting SnH$_4$

Abstract

We studied chemical interaction of Sn with H$_2$ by X-ray diffraction methods at pressures of 180-210 GPa. A previously unknown tetrahydride SnH$_4$ with a cubic structure (${fcc}$) exhibiting superconducting properties below ${T}$$_C$ = 72 K was obtained; the formation of a high molecular ${C2/m}$-SnH$_{14}$ superhydride and several lower hydrides, ${fcc}$ SnH$_2$ and ${C2}$-Sn$_{12}$H$_{18}$, was also detected. The temperature dependence of critical current density ${J}$$_C$(T) in SnH$_4$ yields the superconducting gap 2$\Delta$(0) = 23 meV at 180 GPa. SnH$_4$ has unusual behavior in strong magnetic fields: ${B,T}$-linear dependences of magnetoresistance and the upper critical magnetic field ${B}$$_{C2}$(T) $\propto$ (${T}$$_C$ - ${T}$). The latter contradicts the Wertheimer-Helfand-Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of ${fcc}$ SnH$_4$ in non-superconducting state exhibits a deviation from what is expected for phonon-mediated scattering described by the Bloch-Gr\"uneisen model, and is beyond the framework of the Fermi liquid theory. Such anomalies occur for many superhydrides, making them much closer to cuprates than previously believed.